The present invention relates to a method of making an electrically conductive polyurethane elastomer, and in particular, to one which is substantially free of bubbles and foreign particles. It has particular application as a bias transfer member in transferring toner from an electrostatographic imaging surface to a receiving surface such as a sheet of paper. It has additional application as a conductive intermediate transfer belt or as a transport member.
In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is image wise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a sheet of support material and permanently affixed thereto.
This process is well known and useful for light lens copying from an original and in printing applications from electronically generated or stored originals.
In a reproduction process of the type as described above, it is common practice today to use a biased transfer member to transfer the developed image from the photoconductor to the final support material such as a sheet of paper. Typically, these biased transfer members take the form of a roll and are comprised of a polyester based polyurethane with an additive to control resistivity such as tetraheptyl ammonium bromide.
U.S. Pat. No. 3,959,574 to Seanor et al. describes additional quarternary ammonium compounds for controlling the resistivity of the elastomeric polyurethanes. The above two cross referenced copending applications describe polyether based polyurethanes containing an asymmetric ionic quarternary ammonium salt which has an extended useful electrical life. The transport belts which may be useful in the practice of the present invention may be similar to those described in U.S. Pat. No. 3,931,090 to Amatangelo and in U.S. Pat. No. 4,314,006 to Lentz et al.
While capable of performing satisfactorily certain difficulties do appear in the manufacture of the rolls and belts. In particular, undissolved particles frequently appear in the polyurethane which causes an imperfection in the urethane which in addition to resulting in nonuniform resistivity also leads to poor transfer properties and poor mechanical strength. Furthermore, bubbles appear in the conductive polyurethane, some of which can only be seen with the aid of a microscope, others of which are large enough to be observed with the naked eye. These bubbles provide the same kind of difficulty as the undissolved particles in the polyurethane namely, poor or nonuniform electrical properties, poor mechanical properties such as durometer, tensile strength, elongation, a decrease in the modulus and a decrease in the toughness of the material.